Natural nuclear fission reactor

A natural nuclear fission reactor is a uranium deposit where analysis of isotope ratios has shown that self-sustaining nuclear chain reactions have occurred. The existence of this phenomenon was discovered in 1972 by French physicist Francis Perrin. The conditions under which a natural nuclear reactor could exist were predicted at the University of Arkansas by Paul Kuroda in 1956. The conditions found at Oklo were very similar to what Kuroda predicted.

At the only known location, three ore deposits at Oklo in Gabon, sixteen sites have been discovered so far at which self-sustaining nuclear fission reactions took place approximately 1.5 billion years ago, and ran for a few hundred thousand years, averaging 100 kW of power output during that time.^[1]

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History

In May 1972 at the Pierrelatte uranium enrichment facility in France, routine mass spectrometry comparing UF₆ samples from the Oklo Mine, located in Gabon, Central Africa, showed a discrepancy in the amount of the ²³⁵U isotope. Normally the concentration is 0.7202%; these samples had only 0.7171% – a significant difference. This discrepancy required explanation, as all uranium handling facilities must meticulously account for all fissionable isotopes to assure that none are diverted for weapons purposes. Thus the French Commissariat à l'énergie atomique began an investigation. A series of measurements of the relative abundances of the two most significant isotopes of the uranium mined at Oklo showed anomalous results compared to those obtained for uranium from other mines. Further investigations into this uranium deposit discovered uranium ore with a ²³⁵U to ²³⁸U ratio as low as 0.440%.

Fission product isotope signatures

Nd

Neodymium and other elements were found with isotopic compositions different from what is customarily found on Earth. For example, natural neodymium contains 27% ¹⁴²Nd; the Nd at Oklo contained less than 6% but contained more ¹⁴³Nd. Subtracting the natural isotopic Nd abundance from the Oklo-Nd, the isotopic composition matched that produced by the fissioning of ²³⁵U.

Ru

Similar investigations into the isotopic ratios of ruthenium at Oklo found a much higher ⁹⁹Ru concentration than expected (27-30% vs. 12.7%). This anomaly could be explained by the decay of ⁹⁹Tc to ⁹⁹Ru. In the bar chart below the normal natural isotope signature of ruthenium is compared with that for fission product ruthenium which is the result of the fission of ²³⁵U with thermal neutrons. It is clear that the fission ruthenium has a different isotope signature. The level of ¹⁰⁰Ru in the fission product mixture is low because of a long lived (half life = 10¹⁹ years) isotope of molybdenum. On the time scale of when the reactors were in operation very little decay to ¹⁰⁰Ru will have occurred.

Other observations led to the same conclusion and on the September 25 1972, the CEA announced their conclusion that self-sustaining nuclear chain reactions had occurred on Earth about 2 billion years ago. Later, other natural nuclear fission reactors were discovered in the region.

Mechanism of the reactors

The natural nuclear reactor formed when a uranium-rich mineral deposit became inundated with groundwater that acted as a neutron moderator, and a strong chain reaction took place. The water moderator would boil away as the reaction increased, slowing it back down again and preventing a meltdown. The fission reaction was sustained for hundreds of thousands of years. Fission of uranium normally produces five known isotopes of the fission-product gas xenon; all five have been found trapped within novel aluminium foams in the remnants of the natural reactor, in varying concentrations. This points to on-again, off-again reactor operation. The specific concentrations of xenon isotopes, found trapped in mineral formations 2 billion years later, make it possible to calculate the specific time intervals of reactor operation: approximately 2 hours and 30 minutes.

It is estimated that secondary enrichment of the uranium in centimeter- to meter-sized veins consumed about six tons of ²³⁵U and elevated temperatures to a few hundred degrees Celsius. Remarkably the non-volatile fission products have only moved a few centimeters in the veins during the last 1.5 billion years. This offers a case study of how radioactive isotopes migrate through the earth's crust; a significant area of controversy as opponents of geologic nuclear waste disposal fear that releases from stored waste could end up in water supplies or be carried into the environment.

A key factor that made the reaction possible was that at the time the reactor went critical, the fissile isotope ²³⁵U made up about 3% of the natural uranium, which is comparable to the amount used in some of today's reactors. (The remaining 97% was non-fissile ²³⁸U) Because ²³⁵U has a shorter half life than ²³⁸U, and thus decays more rapidly, the current abundance of ²³⁵U in natural uranium is about 0.7%. A natural nuclear reactor is therefore no longer possible on Earth.

Another factor which probably contributed to the operation of the Oklo natural nuclear reactor was the fact that uranium is soluble in water only in the presence of oxygen. Increasing oxygen content in the earth's atmosphere helps explain why this natural reactor started operation sometime around 2 billion years ago and not before since the percent abundance of fissionable ²³⁵U was at least 3% or higher at all times before reactor startup. Rising oxygen levels during the aging of earth may have allowed a high enough concentration of uranium to accumulate to enhance the possibility of reactor startup.

When oxygenated water trickled over uranium deposits and picked up a few ppm of uranium, uranium may have been re-deposited and concentrated due to the action of algae and geological placer formations more than once. Other geological action such as uplifting and fracturing of the concentrated uranium ore formations may have caused further concentrations into a compacted vein. Without the new aerobic environment available on earth at the time, these concentrations probably couldn't have taken place.

The natural reactor of Oklo can also be used to check if the fine-structure constant α might have changed over time. Alex Shlyakhter proposed in 1976 to measure the abundance of ¹⁴⁹Sm to estimate the cross section for neutron capture of this isotope at that time and check it against the present value. Measurements show that the constant has not changed. (Isaak, Mark. The Counter-Creationism Handbook. U California Press. 2005:p185.)

Relation to Yucca Mountain and other geologic repositories

The deep geological repository concept involves the encapsulation of used fuel in long-lived engineered containers which are then placed and sealed within excavated rooms in a naturally occurring geological formation at a design depth of 500 to 1000 metres below ground surface.

The ability of natural geologic barriers to isolate radioactive waste is demonstrated by the Oklo reactors. During their long reaction period about 5.4 tonnes of fission products as well as 1.5 tonnes of plutonium together with other transuranic elements were generated in the orebody. This plutonium and the other transuranics remained immobile until the present day. This is quite remarkable because ground water had ready access to the deposits and they were not in a chemically inert form, such as glass.

Thus the only known example of underground nuclear waste disposal was successful over a long period in spite of the characteristics of the site. Such a water-logged, sandstone/shale structure would not be considered for disposal of modern toxic wastes, nuclear or otherwise, although the clays and bitumen present played an important part in containing the material.

The US government assessment of the security of Yucca Mountain for spent nuclear fuel storage, drew comparisons with Oklo.

"And when these deep underground natural nuclear chain reactions were over, nature showed that it could effectively contain the radioactive wastes created by the reactions. No nuclear chain reactions will ever happen in a repository for high-level nuclear wastes. But if a repository were to be built at Yucca Mountain, scientists would count on the geology of the area to contain radionuclides generated by these wastes with similar effectiveness."

References

• A. P. Meshik et al. (2004). "Record of Cycling Operation of the Natural Nuclear Reactor in the Oklo/Okelobondo Area in Gabon". Phys. Rev. Lett. 93: 182302. doi:10.1103/PhysRevLett.93.182302.
• Andrew Karam, The natural nuclear reactor at Oklo, Radiation Information Network, April 2005, [1]
• W. Miller et al.: Geological Disposal of Radioactive Wastes and Natural Analogues. ISBN 0-08-043852-0, PERGAMON (2000)
• Gauthier-Lafaye, et al.: Natural fission reactors in the Franceville Basin, Gabon: a review of the conditions and results of a "critical event" in a geologic system, Geochim. Cosmochim. Acta, 60, 48314852, 1996.
• Neuilly, M.et al.: Sur l'existence dans un passé reculé d'une réaction en chaîne naturelle de fissions, dans le gisement d'uranium, C. R. Acad. Sci., 275D, 1847, 1972.
• Raffenach, J. C., Menes, J., Devillers, C., Lucas, M. and Hagemann, R. (1976). Études chimiques et isotopiques de l’uranium, du plomb et de plusieurs produits de fission dans un échantillon de minéral du réacteur naturel d’Oklo. Earth Planet. Sci. Lett. 30, 94)108.
• Kuroda, P. K., J. Chem. Phys.,25, 781–782; 1295–1296 (1956)
• Petrov, Yu. V., Nazarov, A. I., Onegin, M. S., Petrov, V. Yu., Sakhnovsky, E. G. (2006). "Natural nuclear reactor at Oklo and variation of fundamental constants: Computation of neutronics of a fresh core". Physical Review C 74 (6): 064610.
• R. Bodu, H. Bouzigues, N. Moin and J.P. Pfiffelman (1971). "Sur l'existence d'anomalies isotopiques rencontrées dans l'uranium du Gabon". Comp. Rendus Acad. Sci. Paris 275: 1731.